Coherent GEM CO2 laser RF cable lengths.

[macona]

I picked up a Coherent GEM-50 from a friend. Right know it is missing the RF cable and I know the length of these things are multiples of the operating frequency of the laser (supposedly 100mhz) The manual lists the model numbers and the lengths bit they just don't work out. At best the lengths seem to be around 90mhz figuring a teflon dielectric cable.

Can someone with a Gem tell me the length of their cable and what kind of coax they are using? I really don't want to spend $300 for a cable for a laser I don't even know if it works.

-Jerry

[mixedgas]

I have an OEM GEM RF PSU setting in my garage, I believe it is 76 Mhz on mine ; I do not have a laser head for it.

~
You'll have to factor in the velocity factor of the coax when you cut it. ; I'm stuck at work till Zero Dark Thirty, but I will look when I get home to make sure what the frequency is. It does not have to be perfectly a half wave, not darn even close, but for best performance you want the half wave if that is what they tuned/designed their amplifier output stage for.

~
Most of the rest of the RF world has disposed of that old requirement, it is odd for me to see it again.


~
I'd get a really good grade of cable from a known vendor such as RF Parts, Trimming the coax for a half wavelength multiple sounds like some one has bought into the theory of an old CB myth, but I'm sure their RF engineer has his/her reasons

Wavelength Calculator K7TJO (wxtofly.net)


Edited for math error I made.

[krazer]

I think you are pretty close, the sticker on a 100W gem says 100MHz so it seems likely the 50W is the same. The cable lengths seem right for a velocity factor of 0.76 which seems about right.

[macona]

Yeah, the power supply on this unit says 600w @ 100MHz. I was talking to one of the hams at work and he pretty much agreed with not worrying too much about length. Ill just make up a cable close to one of those lengths and give it a try.

Thanks!

-Jerry

[macona]

I found a chunk of Belden 8267 which crosses to RG-213. Has a N on one side and one of those giant BNCs on the other. Its about 3.54m which should be about right, just need a N connector to put on the other end and I should be good to go.

[mixedgas]

I stand corrected, mine is 100 Mhz.

Steve

[macona]

So I made up a cable and got the laser all plumbed and powered. No luck. It gives a VSWR light meaning the tube is not lighting. Tried a couple different cables and exactly the same thing. The manual says that if one of the test voltages (forward power) is below 2.5v the power supply is bad. It is, but when you try to fire the laser it pulses above that.

So I guess the next step is to try and locate a real cable I can use to verify it is not the cable. Not sure where to go past that.

[krazer]

My gem 100 (with integral power supply) exhibits a similar failure -- I concluded that it needs a regas. I haven't got up the courage to send it out yet, but my plan was to try one of the sketchy regassers who claim they will refurb the tube for $1000.

[macona]

I have seen as low as $850 for a pump and fill.

I just need to first find out if it is the cable, power supply, or head for sure though. I wonder if I can hook the power supply to a 50 ohm load and if I can find someone with one that big around here.

[mixedgas]

Photovac outside Columbus Ohio is the real deal.. Multiple plants, multiple employees. I visited on business. I was suitably impressed.

Steve